问题
I have an OpenGL RGBA texture and I blit another RGBA texture onto it using a framebuffer object. The problem is that if I use the usual blend functions with
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA),
the resulting blit causes the destination texture alpha to change, making it slightly transparent for places where alpha previously was 1. I would like the destination surface alpha never to change, but otherwise the effect on RGB values should be exactly like with GL_SRC_ALPHA
and GL_ONE_MINUS_SRC_ALPHA
. So the blend factor functions should be (As,As,As,0) and (1-As,1-As,1-As,1). How can I achieve that?
回答1:
You can set the blend-modes for RGB and alpha to different equations:
void glBlendFuncSeparate(
GLenum srcRGB,
GLenum dstRGB,
GLenum srcAlpha,
GLenum dstAlpha);
In your case you want to use the following enums:
glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ZERO, GL_ONE);
Note that you may have to import the glBlendFuncSeparate function as an extension. It's safe to do so though. The function is around for a very long time. It's part of OpenGL 1.4
Another way to do the same is to disable writing to the alpha-channel using glColorMask:
void glColorMask( GLboolean red,
GLboolean green,
GLboolean blue,
GLboolean alpha )
It could be a lot slower than glBlendFuncSeparate because OpenGL-drivers optimize the most commonly used functions and glColorMask is one of the rarely used OpenGL-functions.
If you're unlucky you may even end up with software-rendering emulation by calling oddball functions :-)
回答2:
Maybe you could use glColorMask()? It let's you enable/disable writing to each of the four color components.
来源:https://stackoverflow.com/questions/146140/opengl-how-to-do-rgba-rgba-blitting-without-changing-destination-alpha